One of those oddities of the British mind, I'm afraid. You can merrily murder humans -including women and kids - by the thousands in your film and nobody will say a word as long as it's the right classification or shown after the watershed. But any suggestion or hint that an animal might be being made mildly uncomfortable - even if it's an obvious visual effect - and Disgusted of Tonbridge Wells will be jumping up and down, foaming at the mouth and writing to their MP!